The disclosure below sets forth a viscometer which is particularly adapted to measure characteristics of drilling fluid, commonly known as drilling mud. It is a device which is aptly made portable so that it can be used in field locations including remotely located drilling rigs. It carries out a routine in testing for viscosity. The apparatus disclosed herein is an improvement over the structure shown in U.S. Pat. No. 4,299,118 which is assigned to the common assignee of this disclosure. The reference discloses what might be termed a motor drive system using circuitry quite different from the control circuitry and power mechanism set forth in this disclosure.
This disclosure incorporates a pulsed stepper motor. For instance, the TRW Company provides a stepper motor which is sold under the trademark GLOBE. Two hundred voltage pulses are required to rotate this motor one full revolution in the full step mode. There are other brands of pulsed motors which have larger increments of rotation. Whatever the choice of pulsed motors, it is driven by voltage pulses. The pulses are applied in relatively rapid fashion so that it turns smoothly. Smoothing is also obtained through the utilization of first and second flywheels, one being on the pulse motor and the other being a relatively larger pulley for rotating a tubular sleeve. The two are connected together by an elastomeric belt. The belt and relatively heavy flywheels smooth rotation so that the rotating sleeve is observed to rotate smoothly, not jerkily. This stirs and mixes drilling mud smoothly. The use of a pulse motor enables the use of digital drive circuitry. This eliminates feedback circuits for motor speed control. Rather, the position of the rotating tubular sleeve in the drilling fluid is known because the pulse motor faithfully follows the pulses applied to it to carry out the test program. A stored program is placed in a memory, the apparatus preferably having a microprocessor along with "read only memory" and "random access memory" (ROM and RAM hereinafter), thereby enabling the stored program to drive the system. If need be, the stored program can be changed.
The present apparatus further utilizes a shaft which is connected to a non-metallic plumb bob which is submerged in the mud undergoing test. The bob is connected to a support shaft. The upper end of the support shaft connects with a coiled spring which returns the bob to a reference azimuth. The shaft is rotationally connected to a rotational encoder. Its deflection is measured dynamically against a reference azimuth. The rotating tubular sleeve (immersed in the fluid undergoing tests) thus couples torque through the mud or drilling fluid to the bob concentric within the tubular sleeve.
Consider the following test sequence exemplary of the present apparatus in operation utilizing a stored program for carrying out the test described below. After filling a container with the drilling fluid undergoing tests and placing it where the bob is completely submerged, and further submerging the tubular sleeve to a specified depth, the following routine is carried out. First of all, with everything stationary, initial movement is begun with a ramp. The rotational velocity of the tubular sleeve is ramped up to 900 RPM. This speed is held for a specified time interval. Thereafter, it is ramped down to another speed, typically 600 RPM. These two velocities are derived from drilling mud specifications published by the American Petroleum Institute (API). On reaching 600 RPM, the first step in processing of obtained data occurs. Rotation of the bob from the zero or reference azimuth is measured. Rotation of the bob occurs as the speed stabilizes within the same defined limits at 600 RPM. The angular displacement of the bob is sensed from the zero azimuth position and continues to be sensed until it stabilizes.
The motor is driven through a second sequence, namely its speed is ramped down from 600 RPM to 300 RPM. The plumb bob changes its angular position as the sleeve speed slows down. Operation is continued at 300 RPM under program control until the bob azimuth angle has stabilized within some defined limit. The tubular sleeve is then stopped for 10 seconds following the API testing standard.
After the 10 second stop, the motor is run at 3 RPM and the peak angular deflection of the bob is observed as an indication of the gel strength of the drilling fluid. Following this, the motor is again ramped up to 900 RPM for 10 seconds of mixing following the API test standard. The motor is then stopped for a waiting interval of 10 minutes according to the API test standard. After the 10 minute wait, the motor is again run at 3 RPM and the peak bob displacement is again observed as an indication of the gel strength of the drilling fluid. The API test sequence is then completed for this fluid sample. The sequence may be repeated for the same sample as desired or a new sequence begun on a new sample at this point.
The microprocessor in the apparatus has thus measured and stored data which is indicative of the bob angular displacement from reference azimuth at 600 RPM and a separate but similar measurement at 300 RPM. Additional bob peak angular displacements have been obtained after specific shutdown intervals, these being preferably 10 seconds and 10 minutes (per API standards) to provide gel strength measurements. The microprocessor displays the aforedescribed data scaled and adjusted by stored scale factors to indicate measurements of viscosity. As desired, the data can be output onto hard copy or stored in memory.
The foregoing describes the nature of the present apparatus which is briefly summarized as a concentrically arranged bob (a solid cylindrical mass) on a shaft surrounded by a rotating sleeve. The two are dipped into the drilling fluid undergoing test to a specified depth stored in a container. A pulse motor drives the rotating sleeve. The motor in turn is driven by a pulsing circuit gated at specified rates by means of a stored sequence of instructions in Read Only Memory (ROM), and the measurements obtained from the rotatable bob as observed by an optical encoder are available for visual output through a LED display, or in memory, or to a suitably interfaced hard copy output device.